Motor control system



p 1943. c. c. LEVY 2,329,127

MOTOR CONTROL SYSTEM Original Filed Feb. 21, 1940 2 Sheets-Sheet 1 w m TN w w m w mm m w 0 L W 2 MAW 4 8 2 Z w o a u n 6 w, w n 4 4 m 2 o J .2 9M l 9 6 a 0/ E m/ o 9 0 5 2 3 2 a 4 w 0 a. w 6 M 5 x 7 P; 5 0 5 7 w 7 zW a. M 7 1 M I M 4 O O 7 o 0 0 7. a M a a 2 a g 6. all y 4 w a 8 i i, a7 7 m. S 2 Z 8 Q N. N 8 6 T 2 Z 1% W4 0 Wm O mU 6 Ox P. at f Patente dSept. 7, 1943 MOTOR C(NTROL SYSTEM Cyril 0. Low, Pittsburgh, Pa.,assignor to Westinghouse Electric & Manufacturing Company, EastPittsburgh, Pa., a corporation of Pennsylvania Original applicationFebruary 21, 1940, Serial No. 320,122. Divided and this application May29, 19.41, Serial No. 395,810

8 Claims.

My invention relates, generally, to control systems, and it hasreference, in particular, to control systems for reversible motors.

This application is a division of my application, Serial No. 320,122,filed February 21, 1940, and assigned to the assignee of the presentinvention.

Generally stated, it is an object of my invention to provide an improvedcontrol system for a reversible motor which is simple and inexpensiveand yet highly sensitive to control conditions.

More specifically, it is an object of my invention to provide forautomatically controlling a reversible motor by electric discharge meansresponsive to the resultant of opposed variable control voltages andsubstantially constant control voltages.

Another object of my invention is to provide for controlling thedirection and speed of a reversible direct current motor by electricdischarge means responsive to the, differential between voltagesproportional to current and voltage conditions of a load circuit and asubstantially constant control voltage.

A further object of my invention is to provide for controlling areversible direct current motor in accordance with predetermined currentand voltage conditions in a load circuit.

An important object of my invention is to provide for controlling thespeed and direction of a reversible motor by grid-controlled electricdischarge means responsiveto a control voltage which is the resultant ofthe differential between electromotive forces responsive to current andvoltage conditions in a load circuit, and a substantially constantcontrol voltage,

Still another object of my invention is to provide a simple and highlysensitive electric discharge control system for effecting smooth controlof the speed and direction of a reversible direct current feed motor inaccordance with predetermined conditions in an associated alternatingcurrent-load circuit.

Other objects will, in part, be obvious and will,

motor may be controlled by electric dischargemeans, the conductivity ofwhich may be controlled by means of control grids. For this purpose,means may be provided for producing variable, opposed control voltagesresponsive to current and voltage conditions of the electrode circuit,and a control voltage which is substantially constant. By applying thesevoltages to the control grids of the electric discharge means in asuitable manner, the operation of the feed motor may be accuratelycontrolled in accordance with the current and voltage conditions of theload circuit, for striking and maintaining an electric arc underpredetermined conditions and maintaining a predetermined power input tothe arc.

For a more complete understanding of the nature and scope of myinvention, reference may be had to the following detailed description,which may be taken in connection with the accompanying drawings, inwhich:

Figure 1 is a diagrammatic view of a motor control system embodying theprincipal features of the invention;

Fig. 2 is a diagrammatic view of a modification of the system of Fig. 1,and

Fig. 3 is a similar view of a. further modificatlon.

Referring particularly to Fig. 1 of the drawings, the reference numerall0 may denote generally a movable electrode adapted to strike andmaintain an electric arc with the work l2, which may, for example, bepositioned in the cupola M of an electric furnace for the purpose ofbeing melted.

The electrode Ill may be connected to a suitable source of electricpower by means of a load or electrode circuit comprising the conductorsI6 and I8, a stabilizing reactor l9, and the secondary winding 20 of apower transformer 22, the primary winding 24 of which may be connectedto a suitable alternating-current source by means of the conductors 26and 28. Suitable means, such as the switch 30, may be provided fordisconnecting the transformer 22 from the source to deenergize the loadcircuit, and for also deenergizing the control system.

The electrode l0 may be provided with suitable actuating means forfeeding it towards or away from the work l2, such as, for example,

the reversible direct current feed motor 32, having a series fieldwinding 34 and a shunt field winding 36 provided with a rheostat 31 forvarying the field current as desired. The armature 38 of the feed motor32 may be operatively connected for feeding the electrode ID in anysuitable manner, such as by means of the feed rolls 48 which engage theelectrode.

With a view to automatically controlling the energy in the load circuit,suitable means may be provided for controlling the energization of thefeed motor 32, such as, for example, the electric discharge devices 44and 46. For example, the electric discharge devices 44 and 46 may beconnected in opposed relation intermediate to a suitable source ofalternating current and the motor 32, in a well-known manner forrectifying both halves of the alternating current wave and supplying aunidirectional current to the motor 32. Thus, the anodes 48 and 58 ofthe electric discharge devices 44 and 46, respectively, may be connectedto the terminals of the secondary winding 54 of a transformer 56, theprimary winding 58 of which may be connected to a source of alternatingcurrent by the supply conductors 26 and 28. The cathodes 68 and 62 ofthe discharge devices may be energized in any manner well-known in theart, and connected to one side of the armature 38, the other sidethereof being connected to a center tap 64 of the secondary winding 54to provide an energizing circuit for the motor 32.

In order to control the value of the current supplied to the motor 32through the discharge devices 44 and 46, the control grids 66 and 68thereof may be connected together and energized in accordance withcurrent and voltage conditions of the load circuit. For this purpose, acurrent transformer 18 may be provided having its primary windingconnected in the load circuit for applying to suitable rectifying means,such as the rectifying bridge circuit 12, a control voltage which isresponsive to the current in the load circuit. The direct-currentterminals of the bridge circuit 12 may be connected to a controlresistor 15, so as to apply thereto the said direct-current voltage.

Suitable means, such as the potential transformer 18, which is energizedfrom the electrode circuit, may be utilized in connection withrectifying means such as the rectifying bridge circuit 88, for applyingto a control resistor 82 a direct current control voltage which isresponsive to voltage conditions in the load circuit. The controlresistors and 82 may be connected to a common terminal 84 so that thevoltages impressed across the resistors are in opposition. Means mayalso be provided for producing a substantially constant control biasvoltage, such as, for example, the control transformer 86, having aprimary winding 81. connected to the supply conductors 26 and 28, and asecondary winding 88 connected to a control resistor 89 through arectifier circuit 98.

The resistors 15, 82 and 89 may then be connected to apply a controlvoltage to the grids 66 and 68. The control resistor 89 may, forexample, be connected in series circuit relation with the controlresistors 82 and 15, so that the voltages of the control resistors 89and 82 are additive, for applying a control voltage between the controland the cathodes 68 and 62 of the discharge devices. Movable contactmembers 9I, 92 and 93 may be employed for making contact with thecontrol resistors 15, 82 and 89 so that the resultant voltage applied tothe control grids renders the discharge devices sufficiently conductivewhen the current and voltage of the load circuit are of the propervalue, to maintain a predetermined rate consumption of electrical energyin the load circuit.

Changing current and voltage conditions vary the voltages across theresistors 15 and 89 in opposite senses, so that the diflerentialtherebetween is normally proportional to the power in the load circuitand is also particularly sensitive to changes of current and voltage inthe load circuit. If the length of the arc increases from the normalvalue the differential is additive with respect to the voltage of theresistor 89. When the arc-length decreases, the differential becomesnegative with respect to the voltage of the control resistor 88.

In order to provide for reversing the direction of the feed motor 32,suitable means may be utilized for reversing the connections of thearmature 38 to the electric discharge devices 44 and 46, such as, forexample, the reversing switch 94, which is provided with an operatingwinding 96, normally open contact members 94a and 94c, 3nd normallyclosed contact members 94b and With a view to providing forautomatically reversing the direction of operation of the feed motor,the operation of the reversing switch 94 may be controlled in accordancewith current conditions in the load circuit. For example, the operatingwinding 98 of a current responsive control relay 99 may be energized inaccordance with the current in the load circuit by means of a currenttransformer I 88, for efiecting the energization of the operatingwinding 96 of the reversing switch 84 when the current in the loadcircuit exceeds a predetermined value. A manually operable controlswitch I82 may also be provided for controlling the energization of theoperating winding 96 so that the feed motor 82 may be reversed at thewill of the operator, when desired.

In operation, the switch 38 may be closed to connect the powertransformer 22 to the source and eflect the energization of the controltransformers 56 and 86. Since there is no current flowing in the loadcircuit at this time, there is no voltage across the control resistor15. The full secondary voltage of the load circuit is impressed on thepotential transformer 18 so that the voltage across the control resistor82 is a maximum. A voltage, which is the sum of the voltages across thecontrol resistors 82 and 89 is, accordingly, applied to the controlgrids 66 and 68 of the discharge devices 44 and 46 through the circuitextending from the control grids 66 and 68 through conductor I84,movable contact 92, control resistor 82, control resistor 15, movablecontact 9I, control resistor 89, movable contact 93, conductor I85, andconductor I86 to the oathodes 68 and 62. The control grids 66 and 68are, therefore, made positive with respect to the cathodes 68 and 62,and the discharge devices 44 and 46 are thereby alternately renderedconductive when their anodes are positive with respect to theircathodes. A fully rectified direct current is thus supplied to the fieldwindings and armature of the feed motor 32 through the circuit extendingfrom the center tap 64 of the control transformer 56, conductor I88,conductor I89, contact members 94d of the reversing switch 94, currentlimiting resistor II8, armature 38, conductor II2, contact members 94b,series field winding 34, conductor II4, conductor I86, cathodes 68 and62, and anodes 48 and 58 of the discharge devices 44 and 46,respectively, back to the terminals of the secondary winding 54. Theshunt field winding 36 is also thereby energized through conductor I88,rheostat 31, field winding 36 and conductor H4. The feed motor 32 thenoperates to feed the electrode I in the direction of the work I2 at arelatively high speed, as the voltage applied to the control grids 66and 68 is a maximum.

Upon the engagement of the electrode I0 with the work I2, arelativelylarge' current flows in the load circuit, so that the currentresponsive relay 99 operates to effect the energization of the operatingwinding 96 of the reversing switch 94 through the normally open contactmembers 99a. The forward operating circuit of the armature 38 is therebyinterrupted by the opening of contact members 9422 and 94d, and thereverse operating circuit completed through contact members 94a and 940,so that the feed motor 32 reverses and withdraws the electrode I 0 fromthe work I2, producing an arc.

With an increase of arc lengththe current in the load circuit decreases.Upon the reduction of the arc current to a predetermined value, the

current responsive relay 99 returns to the deenergized position, so thatthe reversing switch 94 operates to reconnect the armature 38 foreffecting forward feed of the electrod I0 towards the work I2.

With the flow of current in the load circuit, a voltage is producedacross the control resistor which opposes the voltages of the controlresistors 82 and 89. As the arc voltage varies in the opposite sense tothe arc current, the voltage across the control resistor 82 is reduced.The voltage applied to the control grids 66 and 68, and the currentsupplied to the feed motor 32 are reduced accordingly. The speed of themotor 32 and the rate of electrode feed will then be reduced until thearc is of the proper length. At this point the voltages produced by thecontrol resistors 15 and 82*may balance, so that the control voltage ofthe resistor 89 renders the dis charge devices sufficiently conductiveto feed the electrode I0 at a predetermined rate so as to maintain thearc.

When the arc becomes too short, the voltage across the resistor I5increases and that of the resistor 82 decreases, so that theconductivity of the discharge devices 44 and 46 will be reduced inresponse to the changes in the are conditions, so as to effect theoperation of the feed motor 32 to decrease its speed and the rate offeed, and reestablish the predetermined desired arc conditions.Likewise, when the arc becomes too long, the voltage across theresistors I5 and 82 varies in the opposite sense, so that the speed ofthe motor and the rate of feed of the electrode I0 are increased toreestablish the proper conditions.

Referring particularly to Fig. 2 of the drawings, which illustrates amodification of the system of Fig. 1, the load circuit including theconductors I6 and I8 and the electrode I0 may be energized from asuitable source of alternating current by means of the transformer 22 ina similar manner to that illustrated in-Fig. l. The operation of thefeed motor 32 may be controlled in accordance with both the current andvoltage conditions of the load circuit to both vary the speed anddirection of the feed motor in accordance with variations in thoseconditions.

With a view to controlling the speed and direction of the feed motor,the feed motor 32 may be provided with a separately excited fieldwinding I I6 which may be energized from a suitable source, such as, forexample, the battery II8,,through a suitable field rheostat H9, and thearmature 38 of the feed motor may be energized from the secondarywinding 54 of the transformer 56 having a center tap 64. Suitable meansmay be provided for controlling the value and direction of current flowthrough the armature 38, such as,

for example, the electric discharge devices I26,,

. 38 and the center tap 64 of the secondary winding 54 of the controltransformer 56, while the discharge devices I and I 32 are similarlyconnected to the other terminal of the transformer. The dischargedevices I26 and I32 may function as a full wave rectifier for operatingthe feed motor 32 in one direction, while the discharge devices I28 andI30 function for operating it in the opposite direction.

In order to energize the control grids I34, I36, I38 and I40 of thedischarge devices I26, I28, I30 and I32 for suitably controlling theconductivity of the discharge devices in accordance with current andvoltage conditions in the load circuit, a control circuit I42 may beprovided in which grid control voltages proportional to the current andvoltage of the load circuit and substantially constant bias voltages areproduced. To efiect control of the electrode feed motor 32 in accordancewith current conditions of the load circuit, a current transformer 10having its primary winding energized from the load circuit may beconnected by means of a transformer II to the alternating-currentterminals of rectifying means such as the rectifying circuit I2, thedirect-current terminals of which are connected across a controlresistor I5 in the control circuit, so as to produce a currentresponsive control voltage. A potential transformer 18 having itsprimary winding connected to the load circuit may be connectedto arectifying circuit 80 for applying a voltage proportional to the voltageof the load circuit across control resistor 82 of the control circuit.The reactance I44 and condenser I46 may be connected to the rectifiercircuits I2 and 80 to remove any ripple and vary the characteristics ofthe said circuits.

The control resistors I5 and 82 may be connected at one end by a controlbus I 50 and bridged by relatively high value control resistor I52 sothat the voltage across the resistor I52 is the differential between thevoltages of the resistors I5 and 82. The control grids I34 and I40 maythen be connected to the control resistance I52 by an adjustable contactmember I54, and the control grids I36 and I38 by a contact member I56 sothat opposed control voltages may be impressed thereon to control theconductivity of the discharge devices.

A suitable negative bias may be applied to the grids of the dischargedevices for rendering them non-conductive, by means of control resistorsI60 and I62 in the control circuit, which may be connected at one end tothe control bus I50 and at the other end to the cathodes I64 and I10 andI66 and I 68, respectively, of the discharge devices. The resistors maybe energized through suitable rectifier circuits I12 and I14 fromcontrol transformers I16 and I70, respectively. Rectifier d9 vices I80,I82 and I84 and I86 may be connected in paired and opposed relationbetween the re sistors I60 and I62 and the cathodes of the dischargedevices, so as to effectively insulate the bias circuits of thedischarge devices from the motor circuit and permit a substantiallyconstant bias voltage of a predetermined polarity and value to beapplied to the devices. Thus the discharge devices I23 and I32 maycooperate to fully rectify the alternating current impulses from thetransformer 66 when the voltage or the grids I34 and I40 is oi apredetermined value and polarity, and the discharge devices I28 and I30may likewise cooperate when the voltage of the grids I36 and I38 is of apredetermined value and polarity to produce a direct current of theopposite polarity,

To energize the load circuit and effect operation of the electrode feedmotor control system, the switch 30 may be closed to connect the powertransformer 22 to the source of alternating current and effect theenergization of the control transformers 56, I16 and H8. Since the loadcircuit is open, the potential transformer I8 will be energized, so thata voltage appears across the control resistor 82, while none appearsacross the control resistor I5. Thus, a voltage is produced in thecontrol circuit I42 which may be applied to the control grids I34 andI40 of the electric discharge devices I26 and I32 to overcome thenegative bias voltage, through the circuit from the control grids I34and I40, through conductor I90, movable contact I54, control resistorI52, contact member I92, control resistor 82, control bus I50, controlresistor I62, conductor I94, rectifier device I84, and conductor I36, tothe cathodes HO and I64 of the discharge devices I32 and I26. Thedischarge devices I26 and I32 are thereby rendered alternatelyconductive onopposite halves of the alternating current wave to supply afully rectified direct current to the armature 38 of the feed motor 32,through the circuit extending from alternate halves of the secondarywinding 54, through the discharge devices I26 and I32, conductor I96,armature 38, series field winding 34, and conductor I98 to the centertap 64, so as to operate the feed motor 32 to feed the electrode Itowards the work I2.

Upon the engagement of the electrode I0 with the work I2, the voltage ofthe load circuit is greatly reduced, and a relatively large currentflows therein. Accordingly, the voltage across the control resistor 82is greatly reduced, while a relatively high voltage is produced acrossthe control resistor I5. Thus the polarity of the differential voltageacross the resistor I52 is reversed. The voltage applied to the controlgrids I34 and I40 becomes sufliciently negative so that the dischargedevices I26 and I32 cease to conduct. At the same time a voltage isapplied to the control grids I36 and I38 of the discharge devices I28and I30 through the circuit extending from the control grids I36 andI38, through conductor 200, movable contact I56, control resistor I52,contact member 202, control resistor I5, control bus I50 and thenthrough control resistor I60, conductor 204 and rectifier I82 to thecathode I66 of the discharge device I28, and resistor I62, conductor I94and rectifier I86 to the cathode I68 of the discharge device I30. Thedischarge devices I28 and I30 are thus rendered alternately conductiveto supply a direct current to the armature 38 of the feed motor 32 tooperate the feed motor in the reverse direction and withdraw theelectrode I0 from the work I2 so as to strike an arc.

Immediately after striking the arc, the current in the load circuitcommences to decrease, while the voltage in the load circuit commencesto increase. The voltage across the control resistor I52 and thevoltages applied to the control grids E34, I40 and I36, I38 varyaccordingly. The contact members I92 and 202 may be so adjusted thatwhen predetermined are conditions exist, the voltages across the controlresistors 13 and 62 are equal, the voltage across the control resistorI62 is zero, and the discharge devices I26, I32 and I28, I30 arerendered non-conductive by means of the negative voltage bias applied totheir control grids through the control resistors I60 and I62, so as tostop the electrode feed. By adjustment of the contact members I92 and202, the voltage across the resistor may be varied, if desired, so as torender the discharge devices I26 and I 32 suificiently conductive toeifect the operation of the feed motor 32 to feed the electrode I0toward the work I2 at substantially the same rate at which the electrodeis being consumed, when predetermined are conditions exist in the loadcircuit.

Upon variations in the load circuit conditions from the predetermineddesired conditions, the voltages impressed across the control resistorsI5 and 82 also vary, and the value and the polarity of the voltageacross the control resistor I52 vary accordingly. Thus, if the arcbecomes shorter than a predetermined normal value, the current in theload circuit increases and the voltage of load circuit decreases. Thedifferential between the voltages of the control resistors I5 and 82'and the voltage across the control resistor I52 then becomes effectiveeither to render the discharge devices I28 and I30 conductive to operatethe motor 32 to withdraw the electrode from the work so as to increasethe length of the arc to the desired value, or to reduce theconductivity of the discharge devices I26 and I32 so as to slow down themotor 32 and reduce the rate of feed of the electrode toward the workdepending on the adjustment of the contact members I32 and 202.

If the length of the arc exceeds the predetermined normal value thevoltage across the control resistor 82 increases, while the voltageacross the control resistor I5 decreases, so that the differentialvoltage across the control resistor I52 varies in the opposite sense,and is of the opposite polarity. The discharge devices I28, I32 are thenrendered conductive to operate the motor 32 to feed the electrode I0towards the work so as to shorten the length of the arc, or theconductivity thereof is increased to speed up the feed motor in feedingthe electrode toward the work, depending on the adjustment of thecontact members I92 and 202. Depending on the degree of the variationsin the arc length, the changes in the voltage across the controlresistors I5 and 82 vary accordingly to eifect corresponding changes inthe conductivity of the discharge devices so as to vary the value of thedirect current supplied to the feed motor and vary the speed of the feedmotor and the rate of electrode feed to produce the desired arcconditions.

Referring particularly to Fig. 3, which illustrates a modification ofthe control system illustrated in Fig. 2, the electric discharge devicesI26, I32 and I28, I30 may be connected in paired relation in awell-known manner so as to provide for complete rectification of thealternating current from the separate secondary windings 2I0 and 2I2 ofthe control transformer 2l4, for controllin the operation of the feedmotor 32 in opposite directions. For example, the windings 2I0 and 2I2of the transformer 2 may be provided with center taps 2I6 and 2I8,respectively, and the armature 32 may be connected in series circuitrelation between the center taps and the cathodes of the dischargedevices associated with each winding so as to be energized from thewindings 2 I and 2 I 2 in opposite senses.

The control grids I34, I40 and I33, I33 01' the respective pairs ofdischarge devices may be connected together, and separately energizedfrom a control circuit I42 in the manner much similar to thathereinbefore described in connection with Fig. 2, so as to control theoperation of the feed motor 32 to vary both the rate and direction offeed of the electrode ID to strike and maintain an are between theelectrode and the work I2, as hereinbefore described in detail.

Instead of, however, utilizing a common source of variable controlvoltage, such as in the system shown in Fig. 2, separate sources may beprovided for the control circuit I42 so as to more completely isolatethe grid circuits of the different pairs of discharge devices from eachother. The potential transformer 18 may be provided with a splitsecondary winding, each section being connected to a rectifier circuit80. Similarly, the current transformer may be connected to a step-uptransformer 1| having a split secondary winding, each section of whichis connected to a rectifier circuit 12. Control resistors and 82 may beconnected in bridge relation across the respective pairs of rectifiercircuits 12 and 80 as in Fig. 2. Control resistors 223 may be connectedacross the respective pairs of resistors 15 and 82 to provide anadjustable connection from the respective pairs of grids. Separatesource of bias potential, such as the resistors I60 and I2, which may beenergized from control transformers I16 and I18 through rectifiercircuits I12 and I14, may also be provided for applying a negative biasvoltage to the grids I34 and I40 and I38 and I38, respectively, torender the discharge devices non-conductive.

The operation of the control system of Fig. 3 is otherwise.substantially the same as that of the system of Fig. 2, which has beendescribed in detail. The pairs of discharge devices I26, I 32 and I28, Imay be selectively rendered conductive or non-conductive to control thefeed motor 32 so as to feed the electrode I0 in either direction or holdit stationary, depending on both the current and voltage conditions ofthe load circuit, and the relative adjustment of the contact members 222on the respective resistors 220.

In view of the foregoing description, it will be apparent that I haveprovided a simple and effective control system for a reversible feedmotor adapted to actuate a-movable electrode. Both the speed anddirection of electrode feed motor may be readily controlled inaccordance with conditions in the electrode or load circuit. Since thecurrent and voltage conditions in an electrode or load circuit formaintaining an arc normally vary in the opposite directions, bycontrolling the speed of the electrode feed motor in response tovariations in both the current and voltage conditions of the loadcircuit, a highly sensitive control system is obtained. Control of themotor may be had to maintain a predetermined power in the load circuitin this manner. By utilizing electrode feed motor control means of theelectric discharge type, control of the feed motor in response to thecurrent and voltage changes may be readily and accurately secured. Thus,I have, by my invention, provided a simple and inexpensive method ofaccurately controlling the rate of electrode feed which may be readilyeffected and inexpensively maintained.

Since certain changes may be made in the above description, anddifferent embodiments oi. the invention may be made without departingfrom the spirit thereof, it is intended that all matter shown in theaccompanying drawings or contained in the above description shall beconsidered as illustrative, and not in a limiting sense.

I claim as my invention:

1. A control system for a reversible motor comprising, a transformerhaving a pair of terminals and a tapped connection therebetween, a pairof electric discharge devices having control grids, plates and anodesconnected in anti-parallel to each terminal of the transformer in seriescircuit relation with the motor and the tapped connection, a pluralityof unidirectional current devices connected in opposed relation betweenthe anode and cathode connections at each of the transformer terminals,means including a pair of opposed sources of bias voltage connected inseries relation with points intermediate the opposed unidirectionalcurrent devices at each of the transformer terminals to render thedischarge devices non-conductive, means connecting the grid of one ofthe discharge devices connected to one terminal of the transformer andthe grid of the correspondingly connected discharge device at the otherterminal to a source of variable control voltage, and means connectingthe grids of the other discharge device of each pair for the applicationof a variable control voltage of the opposite polarity.

2. A control circuit for controlling a reversible motor in accordancewith variations in a pair of oppositely varying control voltagescomprising, a plurality of pairs of electric valves having controlelectrodes connected in opposed relation between the motor and a sourceof alternating current, circuit means including a voltage dividerconnected to apply the differential of the variable control voltages tothe control electrodes of said pairs of valves in opposite senses, meansproducing a substantially constant control voltage, and circuit means soconnecting said means to the control electrodes and to the circuit meansas to apply a negative bias voltage to the electrodes with thedifferential between the variable control voltages to selectively renderone or another pair of said valves conductive in opposite direction onlywhen the differential voltage exceeds a predetermined positive value.

3. A motor reversing system comprising, a motor having an armature, aplurality of pairs of inversely connected electric valves havingelectrodes for controlling the conductivity thereof connected betweenthe armature and a source of alternating current, a source ofsubstantially constant control voltage connected to each of theelectrodes for rendering said valves non-conductive, a pair of sourcesof control voltages variable in opposite senses independently of cyclicvariations of the alternating current wave, and circuit means includinga voltage divider connecting said variable control voltage sources inopposed relation and applying the resultant voltage to the electrodes ofthe valves in opposite senses to control the conductivity thereof inresponse to variations in said control voltages.

4. A control system for a reversible direct current motor having a motorcircuit energized from a source of alternating current power withrectifier means connected therebetween having control electrodes forcontrolling the direction of current flow comprising, circuit means forapplying opposed variable control voltages to the control electrodes ofth rectifier means to render them.

conductive under different conditions, and additional rectifier meansconnected between the circuit means and the control electrodes inopposed relation to isolate the circuit means from the motor circuit.

5. A control system ior a reversible direct current motor adapted to beenergized in accordance with variations oi voltage or a plurality ofvariable voltage sources from an alternating current source through aplurality or rectifier means arranged to conduct in opposite directionsand having control electrodes for controlling the conductivity thereofcomprising, a source of substantially constant control voltage connectedto each control electrode to render said rectifier means non-conductive,and circuit means connected to said source of control voltage forsuperimposing opposed direct current control voltages from the sourcesof variable voltage on the control electrodes independent of thealternating current wave.

6. A control system for a reversible direct current motor adapted to beenergized from an alternating current source through oppositelyconnected rectifier devices having control electrodes ior controllingthe conductivity thereof comprising, means for producing a variablecontrol voltage, additional means for producing an oppositeiy varyingcontrol voltage, circuit means including an adjustable voltage dividerconnecting said means in opposed relation to produce a variabledifferential voltage therebetween, control means for applyingsubstantially constant control volt-- ages to the control electrodes torender the rectifler devices non-conductive and means connecting thecircuit means and the control means to apply the diflerential voltage tothe control electrodes in opposite senses to selectively render therectifier devices conductive at diilerent intervals dependent on therelative values or the two variable control voltages.

"I. A motor system comprising, a power transformer having a pair oisecondary windings with center taps, a pair of gaseous discharge devicesconnected to the terminals or each winding ior conducting on alternatehali' cycles oi the alternating current wave when rendered conductiveand having control grids to render them conductive, circuit means forapplyi t a variable control voltage to the grids of one pair of devicesto render them conductive under predetermined conditions, additionalcircuit means for applying a variable control voltage of oppositepolarity to the grids of the other pair of gaseous discharge devices torender them conductive under other predetermined conditions, and areversible motor connected in bridged relation between said pairs ofdischarge devices and the center taps operable under the control of saidcontrol voltages.

8. A motor system comprising, a reversible direct current motorconnected to a source of alternating current power by opposed rectifierdevices having control electrodes for controlling the conductivitythereof, a control circuit including sources oi opposed control voltagesvarying in opposite directions connected in opposed relation and asubstantially constant control voltage, a voltage divider connectedacross said sources opposing one 01' said variable voltages connected toapply the resultant voltage to the control electrode of one of therectifier devices, and an additional control circuit including variablecontrol voltages proportional to the variable control voltages in thesaid control circuit and a substantially constant control voltageopposing the variable control voltage proportional to the other of thevariablecontrol voltages in said control circuit connected in theopposite sense to the control electrode oi the other of the rectifierdevices.

CYRILC. LEVY.

